Formulating suspensions of water insoluble, or sparingly soluble solids and/or liquids in personal care compositions such as shampoos and body washes presents a long-standing problem. Formulators need to be able to suspend a variety of such ingredients. For example oils, anti-dandruff agents, such as zinc pyrithione, hair conditioners including cationic polymers, and opacifiers such as mica are widely used. There is therefore a need to disperse/suspend them in aqueous shampoos and body washes.
We have discovered novel structured surfactant systems that are capable of suspending solid particles and oils without sedimentation, using high foaming surfactant systems which give dense stable foams and good skin feel.
The term “structured system” as used herein means a pourable composition comprising water, surfactant, and optionally other dissolved matter, which together form a mesophase, or a dispersion of a mesophase in a continuous aqueous medium, and which has the ability to immobilize non-colloidal, water-insoluble particles, while the system is at rest, thereby forming a stable, pourable suspension. Surfactants and water interact to form phases that are neither liquids nor crystals; these are usually termed “liquid crystal phases,” or alternatively “mesomorphic phases” or “mesophases.”
The term “pourable” is used herein to refer to shear thinning fluids having viscosities around 2000 cps (Brookfield RVT viscometer, spindle 5, speed 100) at room temperature.
Attempts to solve the problem of dispersing water insoluble materials in water have generally involved either using gums or other polymeric thickeners to raise the viscosity of the liquid medium, or else forming colloidal dispersions. More recently the use of lamellar structured surfactants has been proposed.
Gums and polymeric thickeners, which increase the viscosity of the liquid medium, retard, but do not prevent sedimentation, and at the same time make the composition harder to pour. They do not provide stable suspensions.
Colloidal dispersions are prevented from sedimenting by Brownian motion. Such systems are usually incapable of dispersing relatively coarse particles.
Lamellar structured suspending systems depend on the rheological properties of the suspending medium to immobilize the particles, rather than size of the particles. This requires the suspending medium to exhibit a significant yield point that can counteract the sedimenting or creaming of the suspended particles, but is low enough to enable the medium to flow under externally imposed stresses, such as pouring and stirring, like a normal liquid. The structure reforms sufficiently rapidly to prevent sedimentation, once the agitation caused by the external stress has ceased.
Lamellar structured systems all involve the L[alpha]-phase, in which bilayers of surfactant are arranged with the hydrophobic part of the molecule on the interior and the hydrophilic part on the exterior of the bilayer (or vice versa). The bilayers lie side by side, e.g. in a parallel or concentric configuration, sometimes separated by aqueous layers. L[alpha]-phases (also known as G-phases) can usually be identified by their characteristic textures under the polarizing microscope and/or by x-ray diffraction, which is often able to detect evidence of lamellar symmetry. Such evidence may comprise first, second and sometimes third order peaks with a d-spacing (2[pi]/Q, where Q is the momentum transfer vector) in a simple integral ratio 1:2:3. Other types of symmetry give non-integral ratios. The d-spacing of the first peak in the series corresponds to the repeat spacing of the bilayer system.
Most surfactants form an L[alpha]-phase either at ambient or at some higher temperature when mixed with water in certain specific proportions. However, these conventional L[alpha]-phases do not function as structured suspending systems. Useful quantities of solid render them unpourable, and smaller amounts tend to sediment.
The main type of structured system used in practice is based on dispersed spherulitic phase. Spherulitic phases comprise well-defined spheroidal bodies, usually referred to in the art as spherulites, in which surfactant bilayers are arranged as concentric shells. The spherulites usually have a diameter in the range 1000 to 15000 angstroms and are dispersed in an aqueous phase in the manner of a classical emulsion. Spherulitic systems are described in more detail in EP 0 151 884.
Most lamellar structured surfactants require the presence of a structurant, as well as surfactant and water in order to form structured systems capable of suspending solids. The term “structurant” is used herein to describe any non-surfactant, capable, when dissolved in water, of interacting with surfactant to form or enhance (e.g. increase the yield point of) a structured system. It is typically a surfactant-desolubiliser, e.g. an electrolyte. However, certain relatively hydrophobic surfactants such as isopropylamine alkyl benzene sulphonate can form spherulites in water in the absence of electrolyte. Such surfactants are capable of suspending solids in the absence of any structurant, as described in EP 0 414 549.
A major problem with lamellar suspending systems, from the point of view of the formulator of personal care products, is that they are formed most readily by surfactant systems that operate as detergents. The higher foaming surfactants (which are most effective in personal care products, such as shampoos) are more soluble in water and are classed as solubilizers.
Attempts to form stable lamellar suspending systems with high foaming surfactant systems have entailed the use of high concentrations of surfactant and high levels of structurant, such as electrolyte or sugar.
In general the use of high surfactant levels, e.g. greater than about 15-20% by weight, is undesirable on grounds both of cost and the potential for producing adverse effects on skin or hair. High electrolyte levels are similarly undesirable, for their potential effects on skin and hair.
Spherulitic surfactant systems induced with high levels of electrolytes are opaque and this limits the visual effects that can be achieved and may be perceived as less attractive than a clear system in some applications. This is because light cannot pass through the dense matrix of spherulites. By contrast many liquids and gases are transparent because light can pass more readily between the large spaces between their atoms. Many crystals are both solid and transparent, this is because the atoms of a crystal are arranged in a precise lattice structure, stacked in regular rows, with regular spacing between them and hence there are many pathways that a light beam can take through a crystal lattice. Solids can also become transparent if their atoms are arranged randomly, glass and sugar candy are good examples.
Transparency can be obtained in certain spherulitic surfactant systems by adding high levels of soluble carbohydrate (e.g. sucrose); but the sugar generally needs to be present in undesirably high concentrations, e.g. over 20% to be effective. Such systems are described in more detail in US2004235702.
There is therefore a need, especially in the personal care field, for a suspending system that contains a blend of high foaming surfactants and is transparent and mobile; but which does not require the presence of electrolyte or sugar as a structurant.